No document type declaration; implying ...
Cause:
The checked page did not contain a document type (DOCTYPE Declaration).
The validator has tried to validate with a fallback DTD,
but this is quite likely to be incorrect and will generate a large number
of incorrect error messages. It is highly recommended that you insert the
proper DOCTYPE declaration in your document -- instructions for doing this
are given above -- and it is necessary to have this declaration before the
page can be declared to be valid.
Example:
 | <html>
<head>
<title>document without DOCTYPE</title>
</head> ... |
 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>XHTML-document</title>
</head> ... |
 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>XHTML-document</title>
</head> ... |
 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<title>HTML4-document</title>
<p>Some content
|
 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
<head>
<title>HTML-document</title>
<head> ... |
 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html lang="en">
<head>
<title>XHTML11-document</title>
<head> ... |
For complete compliance, a XHTML1.1 document must not be sent as text/html, but as application/xhtml+xml or text/xml as Internet Explorer does not understand the application/xhtml+xml content type and invites the user to save the document instead of rendering it in the browser.
Solution:
Please insert a DOCTYPE declaration as the very first thing in an HTML document.
References: